1. Technical Field Text
This application relates to systems for determining material properties of a sheet dielectric and more particularly to systems for determining material properties of a sheet dielectric using terahertz radiation.
2. Background Information
Sheet dielectrics, such as paper may have one or more material properties that may need to be determined during the manufacture thereof. For example, paper is a thin sheet material typically composed of compressed fibers. The material is produced by pressing together moist fibers into a sheet of a usually uniform thickness and then drying the material. The fibers are usually cellulose pulp made from wood, fabrics, or other vegetable matter. Additives may be incorporated such as chalk, clay, and titanium dioxide. “Sizing” additives may be incorporated to modify the surface absorbency to ink or water, for example, to prevent “bleeding.”
Papers are characterized by several physical parameters. The thickness of paper is referred to the “caliper”. An approximate range of thicknesses is 70 microns (2.76 mils) to 180 microns (7.1 mils). A micron is 0.001 millimeter. A mil is 0.001 in. Cardstock and cardboard may be thicker. Paper is also characterized by its “basis weight” which is a density related to the mass per unit area, typically grams/meter2. A typical range of printing paper is 60 g to 120 g. Heavier paper is considered card stock. The mass per unit area may also be expressed as the weight of a ream of 500 sheets of a standard size of paper. The density of paper (basis weight/caliper) ranges from about 250 kg/m3 to 1,500 kg/m3. Typical printing paper is 800 kg/m3.
Industrial papermaking machines produce a continuous sheet of paper, known as the paper web, starting with wet pulp and ending with the finished dry paper roll. The first step is to deposit the pulp slurry in the forming section. The forming section established the orientation of layup of the fibers, called the “formation.” The press section squeezes the paper web through larger rollers to remove much of the water. The drying section passes the paper web through a serpentine of heated rollers. The water content is reduced to a range of 2% to 10% (typically about 6%) depending on the type of paper. The percentage water content is defined as the weight of the adsorbed water alone divided by the total weight of the water and the paper. The calendar section smoothes the dried paper by pressing the sheet with heavy polished steel rollers.
Paper making machines are typically instrumented with one or more gauges, also referred to as sensors, to measure one or more of the typical characteristics such as caliper, basis weight, formation, and water content. These gauges may be placed after the calendar section to measure the finished properties of the paper; or earlier in the forming, press, and/or drying sections. These properties are used to both characterize the paper and to provide feedback to adjust the paper making machine to produce paper with the desired characteristics.
The most rapid feedback is achieved when the gauges are mounted on-line to directly measure the paper web as it moves through the machine. Ideally, on-line measurement gauges should not disturb or contact the paper web. This requirement restricts the technology used in the gauges to that which does not consume or alter the sample during the measurement process. Historically, basis weight was measured by radiological source gauges, such as beta-gauges, which measures the attenuation of the flux of radiation through the paper. Formation may be measured by vision systems, such as cameras. Water content historically was measured by near infrared (“NIR”) spectroscopy; or measured by microwaves in a resonant cavity. Caliper was (and is) difficult to measure, although gauges employing feelers, laser displacement, or chromatic aberration analysis have some success. On-line gauges may be used for off-line laboratory analysis of paper samples.
The web width of paper-making machines may be several meters wide. However, most on-line gauges most readily only measure a single point or small area of the paper-web at a time. To make measurements along the entire width, the on-line gauges are typically mounted on a motion-control gantry suspended above (and/or below) the width of the web. The motion gantry is programmed to move the gauges laterally across the width of the web (called the “cross-direction” or CD) while the paper web moves underneath the gantry at a steady rate (called the “machine-direction” or MD). Often more than one gauge (sensor) is mounted on the same carriage traversing the gantry. The gantry typically has a position encoder to determine the location of the gauge carriage over the paper web. The gauges on the gantry then essentially trace a zigzag pattern over the paper.
Many types of sensors, most notably beta-gauges, require gauge components, such as the emitter and detector, to be mounted above and below the web. In this case a second gantry is used and the motion of the second gauge carriage is synchronized to the primary gantry carriage. Certain types of gauges must move “off-web” occasionally for calibration or normalization, measuring only air. Many gauges are augmented by temperature, air pressure, and air humidity sensors that may be used to correct the measured parameters for change in the gauge calibration as these environmental conditions may vary.
Historically, in order to measure multiple paper characteristic parameters on the web, the on-line measurement system requires multiple gauges, at least one of each type corresponding to each parameter. For example measurement of basis weight and measurement of water content may require both a beta gauge and a NIR gauge